Micro nebulizers have been used to convert liquid samples to fine droplets suitable for analysis. Micro nebulizers provide a useful interface for analytical systems based on techniques such as mass spectrometry (MS), atomic absorption (AA), or inductively coupled plasma (ICP) which cannot directly analyze liquid samples. In such analytical systems, the liquid sample must first be converted to a gas. The ideal conversion would, theoretically, involve spraying the liquid into uniform fine droplets. The uniform fine droplets then would then be dried and converted to a gas suitable for analysis. In practice, uniform fine droplets are difficult to attain. If droplets vary in size, the heat necessary to dry a larger droplet damages the analyte in a finer droplet. Large droplets, if left undried, result in noise and signal interference.
Current nebulizers rely on a concentric microtube arrangement to spray liquid samples into droplets. The inner microtube carries the liquid sample; the outer microtube carries an inert fluid (liquid or gas) used as a sheath fluid. At the exits of the concentric microtubes, the liquid sample and the sheath fluid collide and the liquid sample is broken into droplets by the shearing force of the sheath fluid. Uniform laminar sheath fluid flow is critical to producing uniform size droplets. Any imperfections in the annular region between the inner and outer microtubes forming the sheath fluid flow region create turbulence in the sheath fluid, which translates directly into lack of control of droplet size and uniformity. Such imperfections may be generated, for example, by transition points within the sheath fluid flow region such as at the point the sheath fluid is introduced into the outer microtube.
To compensate for such imperfections in current nebulizer microtubes, nebulizers with microtubes of relatively great length have been used. The increase in microtube length (in some cases up to 25 mm or more) permits the sheath fluid to stabilize after the turbulence induced by internal imperfections in the sheath fluid entry point transition. However, increased microtube length alone fails to solve the problem entirely or even satisfactorily. Long microtubes dissipate the energy needed for the shearing force collision of sheath fluid and the liquid sample. More problematic is that long concentric microtubes do not stay centered relative to each other; thus, the exit aperture experienced by the sheath fluid is either asymmetrical, changes with time, or both. As a result, the velocity and shearing force of the sheath fluid experienced by the liquid sample is unevenly distributed and changes with time, which brings about the problem that plagues current nebulizers: namely, variation in size and uniformity of the droplets produced.
What is needed is a nebulizer that reproducibly generates uniform fine droplets of controllable size and distribution. Further, what is needed is a nebulizer wherein the aperture experienced by the sheath fluid is controllable. Also desirable is a nebulizer wherein the inner microtube or needle is mechanically stabilized and wherein such stabilizer elements do not substantially impede the sheath fluid flow in the outer microtube. Further, what is needed is a nebulizer wherein the sheath fluid flow path is sufficiently short and smooth such that the reduction of energy associated with liquid droplet formation occurs substantially at or near the point of nebulization.